Once hijacked, the TVs could be made to send messages on behalf of attackers, find other vulnerable devices in a home or launch other attacks across the net.

Detecting and stopping the attack would be difficult, said the researchers.

The attack uses the Hybrid Broadcast Broadband TV (HbbTV) standard that is widely supported in smart television sets sold in Europe.

The HbbTV system was designed to help broadcasters exploit the internet connection of a smart TV to add extra information to programmes or so advertisers can do a better job of targeting viewers.

But Yossef Oren and Angelos Keromytis, from the Network Security Lab, at Columbia University, have found a way to hijack HbbTV using a cheap antenna and carefully crafted broadcast messages.

“For this attack you do not need an internet address, you do not need a server,” Mr Oren told Forbes. “You just need a roof and an antenna and once you are done with your attack, there’s completely no trace of you.”

By exploiting loopholes in HbbTV, smart TVs could be hijacked by attackers and used to do anything their real owners would do.

For instance, the researchers said, if owners had logged in to Facebook via a TV app, the attack could be used to post messages on the social network on that person’s behalf.